Via pourmecoffee, came across a map of electoral results by county from the 1880 presidential election. Because the red-is-Republican/blue-is-Democrat association is fairly new, it's coded with red representing Democrats and blue, Republicans.
I wanted to compare this to the 2012 results, but needed a map showing the colors in the opposite way. So I made one. Below is a map of the 2012 results (missing a few states that aren't in The Guardian dataset I'm using), with the colors as we expect them reversed. The more red, the more strongly the county went for Obama. (The values in Maine and the Northeast are oddly stacked; that's a bug.)
Hit the button, though, and you can switch between the two elections, 132 years apart, and see how counties changed, or didn't.
Note: Click the green tabs on the 2012 view to close the annoying menus.